Source: Guildmaster's Guide to Ravnica

Armor (medium or heavy, but not hide), rare

This suit of armor is reinforced with a magically enhanced metal alloy called mizzium, which is made in Izzet foundries. While you're wearing the armor, any critical hit against you becomes a normal hit. In addition, when you are subjected to a magical effect that allows you to make a Strength or Constitution saving throw to take only half damage, you instead take no damage if you succeed on the saving throw.